Electron microscopes and micro-analysers

ABSTRACT

In a scanning electron beam instrument also capable of being used to exhibit pseudo-Kikuchi (electron channelling) effects by causing the electron beam to rock angularly or spirally about a substantially fixed point on a specimen surface the electron beam is switched rapidly between the rocking (electron channelling) mode and the linear scanning (micrograph) mode to allow the simultaneous display of information derived from both modes.

United States Patent [191 Joy et al.

[ Jan. 15, 1974 ELECTRON MICROSCOPES AND MlCRO-ANALYSERS [75] Inventors:David Charles Joy, Southampton;

Graham Roger Booker, Woodstock, both of England [73] Assignee: CambridgeScientific Instruments Limited, Cambridge, England [22] Filed: Feb. 14,1972 [21] Appl. No.: 225,968

[30] Foreign Application Priority Data Feb. 12, 1971 Great Britain4,604/71 [52] US. Cl... 250/398, 250/311 [51] Int. Cl. H0lj 37/26 [58]Field of Search 250/495 R, 49.5 A,

[56] References Cited UNITED STATES PATENTS 3/1970 Fujiyasu et al.250/495 E OTHER PUBLICATIONS Electron Channelling Patterns From Small(10mm) Selected Areas in the Scanning Electron Microscope Essen et al.,Nature, Vol. 225.

Selected Area Channelling Patterns in Scanning the Electron MicroscopeSchulson et al., 1. Materials Science (1969).

Optimum Conditions for Generating Channeling Patterns in the ScanningElectron Microscope" Schulson et al., J. Sci. lnst. (1969).

Primary ExaminerJames W. Lawrence Assistant Examiner-B. C. AndersonAttorney-Samuel Scrivener, Jr. et al.

[5 7 ABSTRACT In a scanning electronbeam instrument also capable ofbeing used to exhibit pseudo-Kikuchi (electron channelling) effects bycausing the electron beam to rock angularly or spirally about asubstantially fixed point on a specimen surface the electron beam isswitched rapidly between the rocking (electron channelling) mode and thelinear scanning (micrograph) mode to allow the simultaneous display ofinformation derived from both modes.

8 Claims, 4 Drawing Figures ELECTRON MIC ROSCOPES AND MICRO-ANALYSERSThe invention relates to scanning electron microscopes and X-raymicro-analysers in which a finely focussed beam or probe of electrons iscaused to impinge on a specimen to be examined and is caused to scan aregion of the specimen, while an electrical signal derived from theresultant effect on the specimen, for example from a detector of theemitted secondary electrons, back-scattered primary electrons, X-rays,or fromthe specimen current is used to control the brightness of thetrace on the screen of a two-dimensional recorder, in particular acathode-ray tube, which is scanned in synchronism with the scanning ofthe primary electron beam. Thus there appears on the screen of thecathode-ray tube an image of the scanned region of the specimen surfacein terms of the effect on the specimen of the primary beam in producingsecondary or back-scattered electrons, X-rays or specimen current.

Where the primary beam scans a small area of the specimen, for example300 micrometres square, in a two-dimensional raster, the overallmagnification of the microscope is the ratio of the dimensions of theimage on the screen on the cathode-ray tube to the dimensions of thescanning raster. It is well-known to display two images on twocathode-ray tube screens simultaneously, for example one displaying theimage derived from a detector of the X-rays, the other displaying animage derived from the secondary electrons. It has also been proposed inU.S. Pat. Specification No. 3,502,870, based on Japanese Application42/42,790 and 42/42,792 to switch the scanning system acting ontheprimary beam, rapidly between two different amplitude levels, so thatalternate small magnification and large magnification images areproduced in rapid succession and they are displayed side by side on thescreens of two cathode ray tubes, or on different halves of one screen.Thus one image shows a relatively large area of the specimen surface atlow magnification, and the other image shows at much greatermagnification a selected region within that area. In U.S. Pat.Specification No. 3,235,727 it has been proposed, in a scanning X raymicro-analyser, to display simultaneously a two-dimensional image and animage from a slow line scan along a selected path on the specimensurface within the scanned area. Again this is achieved solely bycontrol of the deflection circuits that control the scanning of theprimary electron beam. Both in this case and in the case of the otherU.S. Patent mentioned above, the means for controlling the formation andfocussing of the primary electron beam are undisturbed.

In, recent years it has been found by D.G. Coates (Phil. Mag. No. 16,page 1179, 1967) and others that so-called electron channelling effectsare observable when a finely focussed electron beam is caused to impingeon the surface of a crystalline specimen at varying angles of incidenceand the resulting back-scattered electrons are detected. This effect isanalagous to the Kikuchi effect observed in transmission electronmicroscopes and has been called a pseudo-Kikuchi effect. It has beenproposed to observe this effect by rocking the specimen about twoorthogonal axes intersecting the axis of a fixed electron beam at thepoint of impact. However better results are obtained more effectively bySchulson and van Essen (J. Sci. Instruments 1969) by leaving thespecimen fixedand causing the beam to rock rhythmically about the pointof impact. If the beam is rocked in two mutually perpendicular angulardirections or in a conical path to sweep out a solid angle centred onthe point of impact, and if the resulting back-scattered electron signalis used to control the brightness of the trace on a cathode-ray tubescreen of which the beam is deflected in two perpendicular lineardirections, or in a spiral scan, in synchronism with the angulardeflection of primary beam, a two-dimensional pattern is obtained,displaying the electron channelling or pseudo-Kikuchi effect, andinformation about the crystal structure of the specimen at the point ofimpact can be deduced from this pattern.

It is possible to produce the required rocking primary beam bymodification ofa known scanning electron microscope one such arrangementdescribed by van Essen, Schulson and Donaghay in Nature, Vol 225, No.5235 pages 847-848. However a problem in the practical use of such aninstrument is to identify the point of impact, or to select a particularpoint of impact for examination within a given region of the specimensurface.

According to the invention we now propose, in a scanning electron probeinstrument, to switch the mode of operation of the primary electron beamrapidly back and forth between the 'normal twodimensional rasterscanning mode and the rocking,

channelling-effect, mode and to display simultaneously andside by side(oreven superimposed) the two images derived from these two modes. Theswitching may be done at any suitable frequency and most conveniently itis at the frame frequency, so that one complete two-dimensional frame isproduced, to give an image of a selected area of the specimen, and thisis followed by one complete angular scan with the beam sweeping out asolid angle centered on a fixed point of impact within that area.

It will be appreciated that in contrast to known twinimage displays,this is not merely a matter of altering the scanning;it is necessary toalter the mode of the primary beam. During one frame it behaves as anormal scanning electron microscope beam, then during the next frame italters its behaviour and instead rocks about the point of contact on thespecimen. This involves switching the current in at least onebeamdeflecting or focussing coil, as will become apparent from thedescription below of two preferred embodiments.

The invention will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 is a diagrammatic representation of a normal scanning electronmicroscope operating in the micrograph mode;

FIG. 2 shows the primary beam portion of the microscope of FIG. 1operating in the beam-rocking or electron channelling effect mode;

FIG. 3 shows the instrument of FIG. 2 operating in a modified micrographmode; and

FIG. 4 is a block circuit diagram showing a circuit for switchingbetween the modes of FIGS. 1 and 2 or 2 and 3.

Referring first to FIG. 1, the normal scanning electron microscopecomprises an electron gun G from which the emerging electrons are formedinto a finely focussed beam or probe P by means of a system ofelectromagnetic or electrostatic lenses comprising, in the exampleshown, three electromagnetic lenses L1, L2 and L3. The lenses L1 and L2form an image of the source at a point P1 and this is demagnified by thelens L3 to form a fine spot, only one or two micrometres in diameter, atthe surface of the specimen. The beam impinges on the surface of aspecimen S and the backscattered electrons or secondary electrons aredetected by a detector D which produces an electric signal dependent ontheir quantity. Alternatively the electron beam current flowing to thespecimen itself is measured. An X-ray micro-analyser is similar exceptthat the detector D is replaced by a detector of X-rays.

Two pairs of upper and lower scanning coils, of which the pair acting ononly one plane are visible in FIG. 1 at UC and LC, cause the beam toscan over an area of the specimen in the manner of a television raster,this area being for example of 300 micrometres square, scanned in 300lines, in a frame period which may last anything from a fraction of asecond to several minutes (in the case of an X-ray micro-analyser). Thepurpose of using two pairs of coils UC and LC is to allow the beam, asit scans back and forth, to pass always through the small exit apertureA1 of the final lens L3. This aperture must be small to keep down thespherical aberration, and the short focal length allows insufficientroom to place scanning coils below this aperture.

The deflection current for the coils UC and LC in each scanningdirection is provided by a sawtooth waveform timebase generator TB. Itwill be understood that there will be one such generator for the framescan and a second (not shown) for the line scan in a perpendiculardirection. These generators also control the deflection in correspondingplanes in a cathode-ray tube C, in which the brightness of the trace iscontrolled by the signal from the detector D. Thus there is produced onthe screen of the tube C an image of the scanned region in terms of thesecondary electrons, specimen current, X-ray signal or other effect.

To display the electron-channelling effect it has already been proposedto modify such an instrument by switching off the lower scanning coilsLC, adjusting the lens current in the lenses L1 and L2 to shift theimage of the electron source to a point P2 (FIG. 2) in the plane of theupper scanning coils UC and adjusting the current in the lens L3 as wellso that the demagnified image of the point P2 is still at the specimensurface. Then when the beam is deflected away from the axis of theinstrument (which is also the axis of the lens L3) by the coils UC it isrefracted back onto the axis by the lens L3, so that the lens, as wellas forming a focussing lens, also takes part in the scanning. Neglectingspherical and chromatic aberrations, the overall result is that the beamrocks back and forth about its point of impact on the specimen, as shownin FIG. 2. This forms the subject of British Patent Application No.2829/70 and 32059/70 (cognate) and the corresponding US. Pat.Application No. 108,408 of von Essen et al now US. Pat. No. 3,702,398German P 21 02 616.4 and Japanese 1528/71. To limit theangular-divergence of the beam an aperture A1 is inserted above thescanning coils.

It will be understood that in the mode illustrated in FIG. 2 the beamwill sweep out an angular raster over a square-section solid angle andthe pattern produced on the screen of the cathode-ray tube C willdisplay the effects, on the secondary electron emissionor other detectedeffect, of varying the angle of incidence of the beam on a given fixedspot of the specimen surface. In a modification described in theabove-mentioned earlier application the angular scan is done conicallyrather than in a rectangular raster, for reasons connected with thecorrection of aberrations, and the display on the screen of the tube Ccan correspondingly be spiral. The total apex angle of the cone may bebetween 10 and 20.

The present invention lies in switching rapidly back and forth betweenthe micrograph mode, for example as shown in FIG. 1, and theelectron-channelling mode, such as in FIG. 2, sufficiently rapidly to beable to display two continuous or apparently continuous imagessimultaneously on two screens. To make it simpler to switch between thetwo modes we preferably arrange that the first image is formed at thepoint P2 all the time, i.e., in the micrograph mode as well, and thenthe currents in the lenses L1 and L2 do not need to be altered. It isstill necessary to switch the lower scanning coils LC on and off, andalso to alter the value of the deflection current fed to these coils, asthe angular deflection has to be much higher in the rocking mode than inthe micrograph mode. The gain of the amplifier A of the signal from thedetector may need to be changed. Also,of course it is necessary (unlessone wants both images superimposed onone screen) to switch the outputback and forth from one tube to the other.

A suitable circuit for doing all this is shown in block form in FIG. 4.There are two cathode-ray tubes Cl and C2. Although it would bepossible, if desired, to switch between modes at the line scanningfrequency, this switching is preferably done at framefrequency, so thata complete frame is scanned in one mode before switching to the otherslfnecessary, long-persistence screens may be used on the cathode-ray tubesto ensure maintenance of the images. A train of pulses from the flybackof the frame timebase generator TB acts to change over a bistablecircuit B which, through a relay R if necessary, acts on reed relaysRRl, RR2, RR3 and RR4 to switch on and off scanning coils LC, to directthe output of the detector D to the appropriate cathode-ray tube C1 orC2, to alter thescanning deflection current, and to alter the gain ofthe amplifier A. The aperture Al remains in place but is large enough toallow the rocking mode of operation, and the resulting increasedspherical aberration in the micrograph mode is accepted. The aperture A2also remains in place in both modes.

Because of the aberrations mentioned above the electron beam will not,in the rocking mode, examine a spot of only the diameter of the ideallyfocussed probe, which is one or two micrometres; in practice the spotexamined will be between 5 and 10 micrometres across. However, theelectron channelling effect is adequately displayed and,by virtue of thefact that the micrograph; i.e., the two-dimensional image of a region ofthe specimen, and the electron-channelling pattern of a selected spotwithin that region are simultaneously displayed it is possible withoutdifficulty to correlate the appearance and crystal structure. Moreover,as the user transverses the specimen table to examine a larger region ofthe specimen, for example searching for crystalline domains of aparticular nature, the electronchannelling information in respect of thepoint which,

atany given instant, is at the centre of the field of view is beingcontinuously presented to him. Thisreduces very greatly the risk ofoverlooking a domain of particular interest.

Normally the electron-channelling information will be that in respect ofthe point at the centre of the micrograph picture. Howeverit will beunderstood that, by the provision of manually variable biassing controlswhich are switched into and out of the time base generator circuits insynchronism with the other switching, it would be possible to shift theelectron-channelling spot at will to coincide with any desired region inthe micrograph picture.

Although we have described above an arrangement in which the primarybeam is switched over between the condition of FIG. 2 and substantiallythat of FIG. 1 an alternative arrangement now to be described withreference to FIG. 3 is simpler to put into practice and is in factpreferred. FIG. 3 shows the effect of taking the arrangement of FIG. 2and, without altering anything else, increasing the current in the lensL3. This brings the point about whichthe beam rocks upwards above thespecimen, and so at the specimen surface the beam does now scan a finitearea. This effectively results in a straightforward micrograph mode ofoperation,,equivalent to that of FIG. 1. It is true that not only thenodal point of the scanning but also the focus (or rather the circle ofleast confusion) of the lens is now no longer at the specimen surface,andso the lens is now operating in a partially de-focussed condition.However, although there is therefore some loss of picture quality ascompared with the orthodox micrograph mode of FIG. I, this is acceptablefor the purpose of the invention, which is primarily to give the user asimultaneous general micrograph picture of the region in which he isexamining the electron-channelling eflects, and this loss is outweighedby the advantage of the FIG. 3 arrangement in simplicity of the changingover opera tion. The circuit for achieving the changeover can belikethat ofFIG. 4 except that fewer relays are needed. For example therelay RRl could control the switching inand out of the additionalcurrent to the lens L3, and the relay RR2 could switch the detectorsignal between the two cathode-ray tubes C1 and C2. The switching willagain normally be at frame frequency, so that there are alternatecomplete frames in the two modes, and the speed is limited only by theresponse time of the relays (which could be replaced by solidstatecircuits if necessary) and by restrictions imposed by the inductance ofthe winding of the lens L3. In a typicalcase the current in the lens L3for the electronchannelling mode of FIG. 2 is 600 milliamperes and hasto be increased by 30 milliamperes to produce a micrograph scan, in themanner of FIG. 3, of about 300 micrometres amplitude, where the rockingscan is through a total angle of A similar result to that of FIG. 3could be achieved by reducing the current in the lens L3 instead ofincreasing it, and then the nodal point could fall below the specimensurface, However this is less satisfactory as the resulting micrographimage would then be a reversed one, in both directions, so effectivelyit could appear to have been turned through 180 about-theelectron-optical axis as compared with the orthodox micrograph imageobtained by the system of FIG. 1, and this could be confusing.

It will be understood that, both in the version switching between theFIG. 2 mode and the FIG. 1 mode and in that switching between the FIG. 2mode and the FIG. 3 mode, it would be possible to use spiral scanninginstead of cartesian scanning, in fact the instrument could be providedwith switches allowing selection of either form of scanning at will. Aswitch can also be provided to allow the micrograph mode or thechannelling-effect mode of scanning to be held at will, i.e to halt theswitching-over sequence.

Although we have shown a single detector D used for both modes, it willbe understood that there may be separate detectors, with separateamplifier channels leading to the respective cathode ray tubes. They maybe of different form, for example one responding to back-scatteredelectrons and the other to some other effect. There may be additionaldetectors, for example one responding to X-rays, to display an image ofthe X-ray response, at a particular wavelength, of the scanned region.

We claim:

1. A scanning electron beam instrument comprising means for forming abeam of electrons and causing said beam to impinge on a suitably placedspecimen surface in the path thereof, at least one device for detectingthe effect on said specimen surface of the impact thereon of said beam,means for displaying the resulting information, first scanning controlmeans acting on said electron beam and serving to deflect said beamlaterally so as to scan linearly a finite region of said specimensurface, second scanning control means acting on said electron beam andserving to deflect said beam angularly about a fixed point on saidspecimen surface, whereby said beam rocks about said point, andrepetitively operating switching means acting on said first and secondscanning control means to make each of said control means operative inturn, said switching means acting also on said displaying means wherebyinformation from said detecting device resulting from said linearscanning and said rocking are simultaneously displayed.

2. The instrument set forth in claim 1 including two of said detectingdevices and two respective displaying means.

3. The instrument set forth in claim 1 wherein said switching meansinclude means alternately directing information from a single saiddetecting device to each of two separate said displaying means.

4. The instrument set forth in claim 1 wherein said first and secondscanning control means include some elements common to both means.

5. The instrument set forth in claim 4 wherein said first scanningcontrol means comprise a time base generator and associated first andsecond axially spaced deflecting coils, and said second scanning controlmeans comprise said time base generator and first defleeting coils and afocussing electromagnetic lens acting simultaneously as deflectionmeans.

6. The instrument set forth in claim 5 wherein said switching meanscomprise firstly means for switching on and off said second deflectingcoils and secondly means for altering the power of said first deflectingcoils.

7. The instrument set forth in claim 4 wherein said first and secondscanning control means comprise deflection means common to both controlmeans, and means for acting on a part of said! deflecting means to 8.The instrument set forth in claim 7 wnerein said part of said deflectingmeans comprise an electromagnetic lens serving also as focussing meansfor said beam.

1. A scanning electron beam instrument comprising means for forming abeam of electrons and causing said beam to impinge on a suitably placedspecimen surface in the path thereof, at least one device for detectingthe effect on said specimen surface of the impact thereon of said beam,means for displaying the resulting information, first scanning controlmeans acting on said electron beam and serving to deflect said beamlaterally so as to scan linearly a finite region of said specimensurface, second scanning control means acting on said electron beam andserving to deflect said beam angularly about a fixed point on saidspecimen surface, whereby said beam rocks about said point, andrepetitively operating switching means acting on said first and secondscanning control means to make each of said control means operative inturn, said switching means acting also on said displaying means wherebyinformation from said detecting device resulting from said linearscanning and said rocking are simultaneously displayed.
 2. Theinstrument set forth in claim 1 including two of said detecting devicesand two respective displaying means.
 3. The instrument set forth inclaim 1 wherein said switching means include means alternately directinginformation from a single said detecting device to each of two separatesaid displaying means.
 4. The instrument set forth in claim 1 whereinsaid first and second scanning control means include some elementscommon to both means.
 5. The instrument set forth in claim 4 whereinsaid first scanning control means comprise a time base generator andassociated first and second axially spaced deflecting coils, and saidsecond scanning control means comprise said time base generator andfirst deflecting coils and a focussing electromagnetic lens actingsimultaneously as deflection means.
 6. The instrument set forth in claim5 wherein said switching means comprise firstly means for switching onand off said second deflecting coils and secondly means for altering thepower of said first deflecting coils.
 7. The instrument set forth inclaim 4 wherein said first and second scanning control means comprisedeflection means common to both control means, and means for acting on apart of said deflecting means to operate said part at two differentlevels, one in which said beam rocks about a point on said specimensurface, and a second in which said beam rocks about a point displacedalong the axis of said beam from said specimen surface.
 8. Theinstrument set forth in claim 7 wherein said part of said deflectingmeans comprise an electromagnetic lens serving also as focussing meansfor said beam.